The 34-year-old former Mansfield Town, Stoke and Tranmere Rovers marksman has agreed a two-year deal to become an assistant coach after three seasons playing for the Wellington club.
Greenacre says he turned downing playing offers from two other A-League clubs for the chance to work with head coach Ricki Herbert and goalkeeping coach Jonathan Gould and take charge of the Phoenix `A' team.
"You never want to hang up your boots. You think you can play for ever. But it comes to a point where you have to be realistic and look at where your next options are," he said.
"Retiring from playing has probably come a bit earlier than I hoped but this opportunity might not have been on the table in a year's time."
Greenacre, who first signed with Manchester City as a 16-year-old, scored 18 goals in 83 games for the Phoenix.